GIFT  OF 


3  O 


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UNIVERSITY  OF  CALIFORNIA   PUBLICATIONS 

IN 

ZOOLOGY 

Vol.  9,  No.  2,  pp.  85-104,  pis.  10-12  December  28,  191 1 


ON  A  LYMPHOID   STRUCTURE   LYING 

OVER  THE  MYELENCEPHALON 

OF  LEPISOSTEUS 


BY 

*«4 

ASA  C.  CHANDLER 


CONTENTS 

PAGE 

Introduction     85 

Occurrence  and  homologies  87 

In  genus  Lepiftosteus  87 

Absence  in  other  Ganoids  87 

Possible   homology   with   ' '  Saccus  endolymphaticus ' '  in  Protopterus  88 

General    morphology    88 

Histology    90 

Development  93 

18-22  mm.   stages   93 

55  mm.   stage  94 

Summary 95 

Literature   cited    97 

INTRODUCTION 

The  present  paper  is  a  preliminary  report  on  a  peculiar 
gland-like,  lymphoid  structure  discovered  over  the  myelence- 
phalon  of  Lepisostcus  while  dissecting  out  the  brain  in  the  labor- 
atory of  Professor  B.  G.  Wilder,  at  Cornell  University,  during 
the  fall  of  1910.  The  work  was  begun  in  Cornell  University 
during  the  college  year  1910-11,  and  later  carried  on  in  the 
Zoological  Laboratory  at  the  University  of  California. 

Grateful  acknowledgments  are  here  made  to  Professors  C.  A. 
Kofoid  and  J.  Frank  Daniel,  of  the  University  of  California, 

• 

A  Thesis  submitted  to  the  Department  of 
Zoologj'-  in  partial  fulfillment  of  the  require- 
ments for  the  degree  of  Master  of  Science-  in 
the  College  of  Natural  Sciences,  of  t he- 
University  of  California,  May.  1912. 


86  University  of  California  Publications  in  Zoology 

and  to  Professors  IT.  D.  Reed,  B.  F.  Kingsbury  and  W.  A. 
Hilton,  of  Cornell  University,  for  their  supervision  and  assist- 
ance, as  well  as  for  the  material  used,  and  to  Professor  B.  G. 
Wilder  for  his  helpful  advice  and  the  free  use  which  he  allowed 
of  the  specimens  in  his  neurological  collection. 

The  structure  here  described,  which  I  shall  provisionally  call 
the  myelencephalic  gland,  was  discovered  while  dissecting  out 
the  brain  of  a  long-nosed  gar,  Lepisosteus  osseus.  While  dis- 
secting away  the  cartilage  bit  by  bit  in  the  region  of  the  hind 
brain,  there  was  noted  a  deeply  pigmented  mass  of  tissue  lying 
over  the  myelencephalon,  and  directly  behind  the  cerebellum. 
The  first  inclination  was  to  tear  this  off  with  the  cartilage  and 
dura  mater  as  merely  a  pigmented  mass  of  connective  tissue,  such 
as  often  fills  the  subdural  space  in  teleosts  (see  Wiedersheim. 
1909,  fig.  200,  p.  294).  Since,  however,  it  was  seen  to  have  a 
rather  definite  form,  and  to  be  in  closer  relation  to  the  brain 
than  to  any  other  part  of  the  head,  it  was  left  in  position,  and 
carefully  dissected  out  with  the  brain,  to  the  pial  covering  of 
which  it  was  firmly  attached. 

Though  many  eminent  and  competent  scientists  have  worked 
over  the  brain  of  Lepisosteus,  the  myelencephalic  gland  seems 
almost  universally  to  have  been  overlooked.  The  only  reference 
to  it  which  could  be  found  in  the  literature  on  the  brain  of  the 
ganoids  is  by  Herrick  (1891).  In  his  plate  13,  figure  9,  he 
figures  the  dorsal  aspect  of  the  brain  of  Lepisosteus  osseus,  show- 
ing the  structure  in  question  lying  over  the  myelencephalon. 
No  reference  is  made  to  it  in  the  text,  and  in  the  description  of 
the  figure  he  merely  says:  "The  bilobed  mass  lying  behind  the 
cerebellum  is  not  of  a  nervous  character."  But,  as  pointed  out 
by  Wilder  (1891),  the  membranous  parieties  of  the  brain  are 
an  important  morphological  feature  of  the  organ,  and  should  be 
considered  in  any  treatment  of  the  brain  as  a  whole.  As  will 
be  shown  later,  the  "bilobed  mass"  of  Herrick  is  in  direct  con- 
nection with  the  parieties  of  the  brain,  and  therefore  should  not 
be  omitted  from  a  morphological  study  of  the  brain  because  it 
is  not  of  nervous  tissue. 

Parker  and  Balfour  (1882)  carefully  worked  out  the  brain 
of  the  adult  Lepisosteus,  and  demonstrated  the  delicate  thin- 


Chandler:  A  Lymphoid  Structure  in  Lepisosteus  87 

walled  "vesicle  of  the  thalamencephalon,"  which  had  previously 
escaped  notice  and  is  readily  destroyed  in  dissection,  yet  they 
make  no  reference  to  any  gland-like  structure  lying  over  the 
myelencephalon,  and  the  usual^opening  into  the  fourth  ventricle 
is  shown  in  their  figure.  They  also  made  sections  of  the  entire 
head  of  embryos  up  to  the  26  mm.  stage  in  working  out  the 
development  of  the  brain,  but,  since  the  gland  is  not  recogniz- 
able, as  such,  up  to  that  stage,  it  escaped  their  notice  again. 

In  tearing  off  the  gland  in  a  dissection  of  the  brain,  it  is 
impossible  not  to  tear  off  the  posterior  medullary  velum  with 
it,  which  accounts  for  the  foramen  of  Magendie  usually  shown 
in  figures  of  the  brain  of  Lepisosteus,  as  seen  in  the  figures  of 
Busch  (1848),  Mayer  (1864),  Owen  (1868,  vol.  1,  fig.  174), 
Huxley  (1872,  fig.  38),  Wilder  (1875,  pi.  2,  fig.  7),  Parker  and 
Balfour  (1882,  pi.  25,  fig.  47u),  and  Allen  (1907,  pi.  6,  'figs.  11 
and  12).  Kingsbury  (1897,  pi.  6,  fig.  5)  has  a  diagrammatic 
sketch  of  a  cross-section  of  the  myelencephalon  in  which  he  shows 
the  roof  of  the  fourth  ventricle  undisturbed,  but  without  the 
gland. 

OCCURRENCE  AND  HOMOLOGY 

The  possibility  that  this  gland  might  be  an  abnormal  develop- 
ment presented  itself,  and  to  determine  this  point  two  other  speci- 
mens of  Lepisosteus  osseus  were  dissected.  The  same  structure 
was  found  in  each  of  them,  and  was  of  the  same  size  and  shape. 
It  was  also  found  in  Lepisosteus  platystomus,  only  one  specimen 
of  which  was  available.  No  example  of  L.  tristaechus,  or  alli- 
gator gar,  could  be  obtained  for  dissection,  but  on  account  of 
the  similarity  of  this  species  to  L.  platystomus  in  other  respects, 
and  also  because  of  the  very  close  similarity  of  the  gland  in  the 
two  more  divergent  species  examined,  it  is  probable  that  this 
orijan  will  be  found  in  the  third  species. 

It  was  thought  that  some  such  development  would  be  found 
in  other  types  of  ganoid  fishes,  and  in  the  hope  of  finding  some- 
thing at  least  suggestive  of  it,  numerous  specimens  of  ganoid 
brains  in  the  collection  of  Dr.  Wilder  were  examined,  including 
examples  of  Acipenser,  Scaphirkynchus,  Polyodon,  and  Amia. 
In  none  of  these,  however,  could  any  indication  of  it  be  found, 


88  University  of  California  Publications  in 


though  the  possibility  exists  that  in  some  it  may  have  been 
present,  and  have  been  torn  off  in  dissection.  Polyodon  has  a 
very  deeply  pigmented  pial  covering  of  the  brain,  but  it  remains 
thin  and  membranous  over  the  fourth  ventricle.  I  myself  ex- 
posed several  brains  of  Amia  with  the  myelencephalic  gland 
especially  in  mind  and  found  no  trace  of  such  a  gland  there. 
This  was  unexpected,  since  Amia  is  without  doubt  the  form 
nearest  allied  to  Lepisosteus. 

In  an  article  on  the  central  nervous  system  of  Protoptcms 
annectens,  only  the  preliminary  report  of  which  was  accessible 
to  me,  Burckhardt  (1892)  figures  the  dorsum  of  this  dipnoan 
brain  with  a  structure  possessing  numerous  diverticula  lying 
over  the  hind-brain,  which  he  calls  the  "saccus  endolymphat- 
icus."  This  figure  is  reproduced  by  Wiedersheim  (1909,  fig. 
20lA,  p.  296).  In  a  longitudinal  section  of  the  same  brain  (fig. 
202e  in  Wiedersheim)  this  "saccus"  has  seemingly  been  re- 
moved, as  the  myelencephalon  is  covered  only  by  the  much  folded 
choroid  plexus.  Whether  or  not  Burckhardt  's  "saccus  endolym- 
phaticus"  of  Protopterus  is  in  any  way  related  to  the  myelence- 
phalic gland  in  Lepisosteus,  I  am  unable  to  say  at  the  present 
time,  but  judging  from  the  name  and  its  appearance  in  the 
figure,  it  is  highly  improbable.  If  this  should  be  found  to  be 
homologous,  it  is  all  the  more  strange  that  it  does  not  appear 
in  Amia. 


The  general  form  of  the  gland  in  Lepisosteus  osseus  is  not 
bilobed  as  described  by  Herrick  (1891),  but  it  is  trilobed. 
The  main  body  of  the  gland  is  slightly  wider  than  long,  and  is 
thickened  just  cephalad  of  the  middle,  sloping  off  towards  the 
front  and  back,  and  with  a  pronounced  median  sulcus  caudally, 
causing  the  posterior  border  to  be  emarginate.  From  the  antero- 
lateral  angles  there  projects  on  either  side  an  ear-like  lobe,  very 
definite  and  constant  in  shape  and  size,  connected  with  the  main 
body  by  a  rather  slender  neck  (fig.  A).  These  ear-like  projec- 
tions are  entirely  surrounded  by  cartilage,  making  their  dis- 
section rather  difficult,  although  they  readily  hold  their  shape 
and  position  when  freed  from  the  cartilage. 


1911]       Chandler:  A  Lymphoid  Structure  in  Lepisosteus  89 


ant.  I.  cb. 


post,  I.  cb. 


myelen.  gl. 


sp.  cd. 


Fig.  A.  Dorsal  aspect  of  brain  of  Lepisosteus  osseus,  with  myelence- 
phalie  gland  in  situ.  X  10.  ant.  I.  cb.,  anterior  lobe  of  cerebrum;  'post.  I. 
cb.,  posterior  lobe  of  cerebrum;  opt.  1.,  optic  lobe;  trilob.  cbl,  trilobed  cere- 
bellum; myelen.  gl.,  myelencephalic  gland;  sp.  cd.,  spinal  cord. 


90  University  of  California  Publications  in  Zoology 

In  Lepisosteus  platystomus  the  gland  is  almost  identical  in 
form  to  that  in  L.  osseus,  but  the  ear-like  projections  are  rela- 
tively shorter  and  stouter  than  in  any  specimen  of  the  latter 
species  which  I  have  examined. 

The  size  of  the  gland  as  compared  with  the  rest  of  the  brain 
is  considerable.  The  width  of  the  main  body  in  a  25  cm.  speci- 
men is  slightly  greater  than  the  width  of  the  optic  lobes,  which 
form  the  widest  dimensions  of  the  brain,  while  the  width  from 
tip  to  tip  of  the  lateral  lobes  is  more  than  twice  that  of  the 
cerebrum  at  its  greatest  width.  Its  length  is  approximately  that 
of  the  cerebellum,  and  its  thickness  such  that  it  attains  the  level 
of  the  dorsum  of  the  cerebellum.  In  adults  the  size  of  the  gland 
as  compared  with  the  rest  of  the  brain  is  even  greater  than  this. 

The  entire  brain  of  Lepisosteus  is  covered  by  a  deeply  pig- 
mented  membrane  which  has  a  peculiar  metallic  appearance,  in 
which  the  enormous  pigment  cells  appear  under  the  binocular 
microscope  as  a  sprinkling  of  pepper.  This  covering  membrane 
seems  to  be  directly  continuous  with  the  covering  of  the  myelence- 
phalic  gland,  or  with  the  gland  itself,  since  in  microscopic  section 
there  is  no  distinctly  differentiated  outer  covering.  In  other 
words,  the  structure  seems  to  be  a  very  highly  developed  and 
enlarged  portion  of  the  pigmented  covering  of  the  brain. 

HISTOLOGY 

In  order  to  determine  the  histological  structure  of  the  mye- 
lencephalic  gland,  the  best  specimen  available  was  fixed  in  for- 
malin, imbedded  in  paraffin,  and  cut  in  sections  12  microns  in 
thickness.  The  sections  were  then  stained  in  Delafield's  haemo- 
toxylin,  and  counter-stained  with  eosin  or  with  picro-fuchsin. 

When  placed  under  the  microscope,  it  became  at  once  ap- 
parent that  the  structure  under  study  was  not  of  nervous  tissue, 
but  appeared  as  an  enormous  development  of  the  pial  covering 
of  the  brain.  In  some  of  the  sections  there  appeared  to  be  a 
cavity,  but  this  proved  to  be  nothing  more  than  a  very  large 
central  blood  vessel  or  sinus.  In  the  section  drawn  (pi.  10, 
fig.  1)  the  true  pia  mater  is  indistinctly  differentiated  from  the 
gland  across  the  dorsum  of  the  fourth  ventricle  and  is  fused 
with  it.  Where  the  pia  mater  curves  down  over  the  side  of  the 


1911]       Chandler:  A  Lymphoid  Structure  in  Lepisosteus  91 

medulla,  however,  the  tissue  of  the  gland  is  continued  into  it 
as  shown  in  the  region  marked  "A"  in  plate  10.  As  stated 
above,  there  is  no  apparent  differentiated  covering  of  the  gland. 

Of  the  various  histological  elements  present,  the  most  con- 
spicuous are  the  extremely  large  and  irregular,  black  pigment 
cells,  scattered  irregularly  throughout  the  whole  organ.  These 
cells  are  similar  to  those  in  the  pigmented  covering  of  the  brain, 
except  that  they  are  not  so  flattened,  and  send  their  branches 
freely  in  all  directions.  Under  the  high  power  of  the  microscope, 
they  show  the  typical  granular  structure  of  melanin  pigment 
cells,  and  the  granules  are  often  somewhat  scattered  at  the  peri- 
phery, where  the  cell  has  been  cut.  Some  of  these  pigment  cells 
measure  fully  eighty  microns  from  tip  to  tip  of  their  branches. 

In  the  middle  of  the  dorsal  part  of  the  gland  there  is  an  open 
reticulum  of  connective  tissue  which  is  gradually  encroached 
upon  by  the  more  solid  substance  (pi.  10,  fig.  1).  From  the 
microscopical  appearance  of  other  parts  of  the  gland,  there  is 
much  evidence  that  this  reticulum  forms  the  framework  for  the 
entire  structure,  the  other  elements  being  netted  in  it.  The 
appearance  of  this  network  in  a  "solid"  part  of  the  gland  may 
be  seen  in  plate  11,  figure  2,  which  represents  such  a  portion 
highly  magnified.  Farther  cephalad  than  the  region  shown  in 
plate  10,  figure  1,  which  represents  a  section  slightly  in  front  of 
the  middle,  the  network  becomes  more  and  more  open,  until, 
on  the  sides  under  the  cerebellum,  nothing  is  left  but  the  retic- 
ular  connective  tissue  with  a  few  pigment  cells  in  it.  Blood 
vessels  are  of  frequent  occurrence,  running  in  all  directions,  and 
ranging  from  very  large  ones  visible  to  the  naked  eye  to  very 
minute  capillaries.  Even  in  the  open  network  of  connective 
tissue  surrounding  the  hinder  part  of  the  cerebellum,  and  con- 
stituting the  cephalic  portion  of  the  structure  in  question,  blood 
vessels  ramify  quite  freely.  The  pia  mater  on  the  sides  of  the 
medulla,  and  surrounding  the  cerebellum,  is  almost  a  solid  mass 
of  blood  vessels,  which  are  densely  crowded  with  corpuscles,  so 
crowded,  in  fact,  that  they  appear  as  solid  masses  of  tissue,  and 
their  identity  was  for  some  time  in  doubt.  Caudally,  the  open 
network  of  connective  tissue  is  lost  entirely,  and  the  gland 
appears  solid  throughout,  and  denser  than  the  portion  drawn 
(pi.  11,  fig.  2). 


92  University  of  California  Publications  in  Zoology 

The  histologieal  elements  found  in  the  connective  tissue  net- 
work of  the  gland,  in  addition  to  pigment  cells,  are  of  three 
kinds:  (1)  large,  clear  cells  with  small,  deeply  staining  nuclei 
(pi.  11,  fig.  2,  eryth.)  ;  (2)  large  cells  more  or  less  deeply  clouded 
with  blue  in  material  stained  in  haemotoxylin,  and  showing  evi- 
dence of  reticular  chromatin  network  (pi.  11,  fig.  2,  leuc.)  •  and 
(3)  cells  filled  with  masses  of  granules  staining  deep  red  with 
eosin,  and  yellow  with  picro-fuchsin  (pi.  11,  fig.  2,  gran.  m.}. 

The  first  cells  above  mentioned  seem  to  be  erythrocytes,  as 
they  have  precisely  the  same  appearance  as  those  filling  the 
vessels  in  the  pia  mater,  where  they  are  associated  with  fairly 
numerous  leucocytes.  They  are  scattered  freely  throughout  the 
gland,  entirely  independent  of  vessels  of  any  sort.  This  is  very 
remarkable  for  an  animal  which  has  a  closed  blood  system,  and 
no  explanation  for  such  a  phenomenon,  if  they  really  be  erythro- 
cytes, has  yet  been  found.  These  cells  range  from  eight  to  ten 
microns  in  diameter,  are  clear  and  transparent,  with  small, 
round,  deeply-staining  nuclei,  and  are  irregular  in  outline. 
though  this  may  be  due  to  slight  shrinkage  or  contact  with  other 
cells. 

The  second  cells  above  described  have  much  the  appearance 
of  large  leucocytes.  They  are  very  uniformly  round  in  outline, 
and  vary  from  seven  to  nine  microns  in  diameter. 

The  most  peculiar  and  characteristic  element  present,  how- 
ever, are  the  numerous  cells  filled  with  granules.  The  granules 
are  about  one  and  a  half  microns  in  diameter,  and  are  very 
nearly  the  same  in  size  as  the  melanin  granules  in  the  pigment 
cells.  Where  a  pigment  cell  and  a  mass  of  granules  have  been 
cut  in  close  proximity  and  both  types  of  granules  slightly  scat- 
tered, it  is  difficult  to  distinguish  them  except  by  color.  As 
stated  above,  they  stain  a  very  deep  red  with  eosin,  suggesting 
eosinophile  granules  in  leucocytes,  but  they  are  larger  and  occupy 
the  cell  more  completely,  as  figured  by  Rawitz  (1900).  With 
picro-fuchsin,  on  the  other  hand,  they  stain  a  deep  yellow,  similar 
to  the  color  given  to  muscle  fibres.  In  many  of  the  vessels  of  the 
gland,  especially  in  the  larger  ones,  there  are  areas  filled  with  a 
substance  which  strongly  suggests  granules  in  process  of  disin- 
tegration, and  occasional  scattered  granules  still  intact  may  be 


Chandler:  A  Lymphoid  Structure  in  Lepisosteus  93 

found  in  these  areas.  It  is  further  significant  that  the  substance 
stains  exactly  the  same  as  the  granules,  red  with  eosin,  and 
yellow  with  picro-fuchsin.  I  cannot  say  that  the  granules  do 
pass  into  the  vessels  and  disintegrate,  but  there  is  no  positive 
evidence  against  it,  and  there  are  some  facts  in  favor  of  it. 
Although  the  granules  appear  to  be  normal,  the  possibility  exists 
that  they  may  be  due  to  parasitism,  or  some  other  abnormality. 
The  masses  of  granules,  from  ten  to  twelve  microns  in  diam- 
eter, are  held  together  by  some  membrane,  probably  a  very  thin 
cell  wall,  but  it  is  not  evident  in  the  sections.  The  granules  are 
sometimes  scattered  somewhat  when  the  mass  is  cut  across  just 
as  are  the  melanin  granules  in  the  pigment  cells.  Associated 
with  each  mass  there  is  a  fairly  large  nucleus  which  shows  much 
more  plainly  in  some  cases  than  in  others  (pi.  11,  fig.  2).  Due 
to  the  lighter  coloring  of  the  granules  with  picro-fuchsin,  the 
nuclei  show  best  with  that  stain.  These  granular  masses  are 
found  in  varying  density  throughout  the  gland,  except  in  the 
open  connective  tissue  network  immediately  behind  and  around 
the  cerebellum.  In  the  main  body  of  the  gland  they  are  scat- 
tered in  approximately  the  density  shown  in  plate  11,  figure  2, 
though  possibly  on  an  average  slightly  more  numerous.  In  the 
ear-like  projections,  however,  they  are  far  more  dense,  so  dense, 
in  fact,  as  to  obscure  all  the  other  kinds  of  cells,  and  to  conceal 
the  reticular  network  entirely. 

DEVELOPMENT 

Thinking  that  the  origin  and  development  of  this  gland-like 
structure  might  throw  some  more  definite  light  on  its  nature  and 
function,  I  sectioned  a  series  of  embryos  ranging  from  6  mm. 
to  22  mm.  in  length.  The  heads  of  these  embryos  were  stained 
in  toto  in  Delafield's  haemotoxylin,  imbedded  in  paraffin,  and 
cut  in  sections  10  microns  in  thickness. 

As  the  eosin  counter-stain  was  the  most  effective  with  the 
adult,  this  was  likewise  used  for  the  embryos. 

Up  to  the  18  mm.  stage  there  could  be  found  no  indication 
whatever  of  any  structure  lying  over  the  myelencephalon,  the 
ependymal  lining  of  the  brain  cavity  coming  in  close  juxtapo- 
sition to  the  cartilaginous  roof  of  the  skull,  or  lying  immediately 


94  University  of  California  Publications  in  Zoology 

beneath  the  skin  in  the  very  young  specimens  in  which  the 
cartilaginous  roof  is  not  yet  developed. 

In  the  18  mm.  specimen  was  found  the  first  indication  of 
any  tissue  intervening  between  the  brain  covering  and  the  roof 
of  the  skull,  in  the  form  of  a  blood  sinus,  a  space  across  the 
dorsum  of  the  myelencephalon  filled  with  blood  corpuscles.  In 
the  21.5  and  22  mm.  stages  the  condition  is  very  similar  except 
that  the  blood  sinus  is  larger  and  more  conspicuous.  A  typical 
section  through  the  myelencephalon  of  the  21.5  mm.  embryo  is 
shown  in  plate  12,  figure  3.  Between  the  band  of  columnar 
endothelial  cells  covering  the  fourth  ventricle,  and  the  cartilag- 
inous roof  the  skull,  there  is  a  space  largely  filled  with  blood 
corpuscles,  and  with  a  few  strands  of  reticular  connective  tissue. 
A  few  scattered  pigment  cells  may  also  be  seen.  Immediately 
beneath  the  skin,  even  in  those  embryos  where  the  roof  of  the 
skull  is  not  yet  developed,  there  is  a  dense  layer  of  pigment  cells. 
As  the  cartilage  grows  over  the  dorsum,  it  is  easy  to  see  how  sonic 
of  the  pigment  cells  might  be  pinched  off  and  left  inside,  there 
to  multiply  and  cause  the  apparently  useless  pigmentation  of 
the  covering  of  the  brain.  At  any  rate,  it  seems  highly  probable 
that  the  pigment  cells  found  there  are  derived  from  the  pig- 
mented  layer  of  the  integument. 

The  next  embryo  available  for  study  was  a  55  mm.  specimen 
which  was  cut  in  sections  10  microns  in  thickness  and  stained 
with  Delafield's  haemotoxylin  and  erythrosin.  The  condition 
there  presented  is  extremely  instructive,  as  it  is  in  every  way 
intermediate  between  the  22  mm.  stage  and  the  adult  form.  The 
space  between  the  covering  of  the  fourth  ventricle  and  the  skull 
is  much  widened,  being  at  least  as  wide  as  the  depth  of  the 
fourth  ventricle.  This  space  is  largely  empty,  but  is  partially 
filled  by  a  reticulum  of  connective  tissue  (pi.  12.  fig.  4).  In 
this  reticulum  are  large  blood  sinuses,  more  or  less  densely 
crowded  with  corpuscles,  and  with  a  number  of  large,  scattered 
pigment  cells.  The  latter  differ  from  those  in  the  adult  structure 
in  that  they  are  far  more  regular  in  outline,  often  nearly  round, 
and  without  the  dendritic  branches  displayed  later.  It  will  be 
noticed  that  in  plate  12,  figure  4,  there  are  lateral  outpocketings 
of  the  ependymal  epithelium.  Farther  caudad  these  outpocket- 


1911]       Chandler:  A  Lymplioid  Structure  in  Lepisosteus  95 

ings  are  larger  and  much  more  pronounced,  curving  forwards 
or  backwards,  so  that  in  some  sections  they  appear  entirely  sep- 
arated from  the  ependyma,  and  look  like  cavities  lined  with 
epithelium  and  surrounded  by  the  reticular  connective  tissue 
lying  over  the  myelencephalon.  Farther  caudad  than  the  region 
represented  in  plate  12,  figure  4,  also,  the  cavity  over  the  brain 
is  much  extended  laterally,  and  the  lateral  portions  are  largely 
filled  in  with  embryonic  connective  tissue  cells.  These  regions 
seem  to  be  the  centers  of  proliferation  of  the  connective  tissue, 
as  here  the  cells  are  hardly  differentiated,  while  towards  the 
median  line  the  reticular  processes  of  the  cells  become  more  and 
more  developed,  and  the  cells  themselves  become  fewer  in  number. 
At  this  stage  the  gland  has  not  the  definite  outline  which  is 
apparent  in  the  adult,  and  there  is  still  no  indication  of  the 
leucocytes  or  granular  masses  which  form  such  a  conspicuous 
part  of  the  fully  developed  structure. 

It  has  not  been  possible  thus  far  to  obtain  a  specimen  inter- 
mediate between  this  55  mm.  stage,  and  a  young  adult  of  250 
mm.,  the  brain  of  which  is  figured  entire  in  figure  A.  This  spec- 
imen has  not  yet  been  sectioned,  but  as  it  is  adult  in  all  characters 
except  size,  it  is  doubtful  whether  it  will  throw  any  more  light 
on  the  development  of  the  gland.  It  has,  in  this  specimen,  the 
characteristic  shape,  but  is  slightly  smaller  in  proportion  to  the 
brain  than  in  older  individuals. 

* 

Sl'.MMARY 

1.  A  lymphoid,  "gland-like  structure  overlies  the  myelencepha- 
lon  of  Lepisosteus,  and  is  closely  associated  with  the  pial  covering 
of  the  fourth  ventricle.     Though  large  and  conspicuous,  it  has 
been  almost  universally  overlooked  by  workers  on  the  brain  of 
Lepisosteus. 

2.  The^jnyelencephalic    gland    (provisionally    so   named)    is 
present  in  both  Lepisosteus  osseus  and  L.  platystomus,  which  are 
the  only  species  of  the  genus  examined  by  me.     No  indication 
of  it  has  been  found  in  other  ganoids. 

3.  The  size  and  form  are  very  constant  in  all  specimens  dis- 
sected and  are  practically  the  same  in  both  species.    Topograph- 
ically it  seems  to  be  a  highly  developed  portion  of  the  pia  mater. 


96  University  of  California  Publications  in  Zoology 

4.  The  microscopic  structure  is  suggestive  of  a  lymph  gland. 
A  reticulum  of  connective  tissue  underlies  the  whole,  and  in  this 
a  number  of  other  histological  elements  are  netted. 

5.  The  most  characteristic  feature  is  the  abundance  of  cells 
containing  masses  of  granules   staining   deeply  with  eosin,   of 
unknown  nature,  and  resembling  melanin  granules  in  size  and 
form. 

6.  There  is  some  evidence  that  the  granules  mentioned  above 
pass  into  the  blood  vessels  and  there  disintegrate. 

7.  The  earliest  indication  of  the  structure  is  in  an  18  mm. 
embryo,  where  there  is  a  blood  sinus  lying  between  the  covering 
of  the  fourth  ventricle  and  the  cartilaginous  roof  of  the  skull. 
This  sinus  is  larger  and  more  conspicuous  in  embryos  21.5  and 
22  mm.  in  length. 

8.  In  a  55  mm.  embryo  the  condition  is  directly  intermediate 
between  the  22  mm.  stage  and  the  adult.     The  connective  tissue 
reticulum  and  pigment  cells  are  well  developed,  and  large  blood 
sinuses  are  still  present,  but  the  granular  masses  so  characteristic 
of  the  adult  are  not  vet  in  evidence. 


1911]       Chandler:  A  LympJioid  Structure  in  Lepisosteus  97 


LITERATURE  CITED 


ALLEN,  W.  F. 

1907.  Distribution  of  the  subcutaneous  vessels  in  the  head  region  of 
the  Ganoids  Polyodon  and  Lepisosteus.  Wash.  D.  C.,  Proc. 
Acad.  Sci.,  9,  79-125,  pis.  1-15. 

BALFOUR,  F.  M.,  and  PARKER,  W.  N. 

1882.  On  the  structure  and  development  of  Lepisosteus.  Phil.  Trans. 
R.  Soc.,  London,  173,  pt.  2,  359-442,  pis.  21-29. 

BURCKHARDT,   R. 

1892A.  The  central  nervous  system  of  Protopterus  annectens,  preliminary 
report,  J.  Comp.  Neur.,  2,  89-91,  pi.  13. 

1892B.  Das  Centralnervensystem  von  Protopterus  annectens,  8vo.,  64  pp., 
5  pis.  (Friedlander,  Berlin). 

BUSCH,  W. 

1848.     De  selachiorum  et  ganoideorum  encephalo.     (Berlin.) 

HERRICK,  C.  L. 

1891.  Contribution  to  the  comparative  morphology  of  the  central 
nervous  system.  III.  Topography  and  histology  of  the  brain 
of  certain  ganoid  fishes.  J.  Comp.  Neur.,  1,  149-182,  pis. 
11-13. 

HUXLEY,  T.  H. 

1872.  Manual  of  the  comparative  anatomy  of  vertebrates  (New  York, 
Appleton),  431  pp.,  110  figs,  in  text. 

KINGSBURY,  B.  F. 

1897.  Encephalic  evaginations  in  ganoids.  J.  Comp.  Neur.,  7,  37-44, 
pi.  6. 

MAYER,  F.^T.  C. 

1864.  Ueber  den  Bau  des  Gehirns  der  Fische  in  Beziehung  auf  eine 
darauf  gegrundete  Eintheilung  dieser  Thierklasse.  Halle. 
Nova  Acta  Leop.,  30,  40  pp.,  7  pis. 

OWEN,  R, 

1861-68.  Comparative  anatomy  and  physiology  of  vertebrates.  (Lon- 
don, Longmans,  Green).  I.  xlii,  650  pp.,  452  figs,  in  text. 


98  University  of  California  Publications  in  Zoology 

RAWITZ,  B. 

3900.  Ueber  die  Blutkorpercheu  einiger  Fische.  IT.  Ganoiden  und 
Teleostier.  Arch.  f.  mikr.  Anat..  56.  149-168,  pi.  6. 

WlEDERSHEIM,  R. 

1909.  Vergleichende  Anatomic  der  Wirbelthiere.  (Ed.  7,  Jena, 
Fischer),  xx.  935,  1  pi.,  476  figs,  in  text.  Trans,  of  6th  ed. 
by  W.  N.  Parker  (New  York,  Macmillan),  1907,  xii,  576,  372 
figs,  in  text. 

WILDER,  B.  G. 

1875.  On  the  brains  of  Amia,  Lepisosteus,  Acipenser,  and  Polyodon. 
Proc.  Amer.  Assn.  Adv.  Sci.,  24,  168-193.  pis.  2-3. 

1891.  The  morphologic  importance  of  the  membranous  or  other  thin 
portions  of  the  parieties  of  the  encephalic  cavities.  J.  Oomp. 
Neur.,  1,  201-203. 


EXPLANATION  OF  PLATE  10 

Fig.  1.  Cross-section  of  the  myelencephalon  and  myeleneephalic  gland 
of  a  Lepisosteus  osseus,  250  mm.  in  length.  Section  taken  slightly  cepha- 
lad  of  middle  of  gland,  and  cut  farther  cephalad  on  right  than  on  left. 
X  12. 

ABBREVIATIONS 

A — region  where  tissue  of  gland  extends  into  pia  mater. 

4th  vent. — fourth  ventricle. 

bl.  ves. — blood  vessel. 

col.  c. — columnar  epithelial  cells. 

cann.-tiss.  n. — connective  tissue  network. 

d.  gran.  m. — region  dense  with  granular  masses. 

endol. — endolymph. 

fl.  c. — flattened  epithelial  cells. 

inf.  pi. — infolding  of  choroid  plexus. 

med. — medulla  oblongata. 

nerv. — nerve. 

pig. — pigment  cell. 

p..  m. — pia  mater. 


[100] 


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o 

I^^BJBf/ 

a 

a 

BPifc? 

BK/-> 

Co' 
00 

'<  -.':,:> 

a 

u      ''      '         ' 

^  w>    ••'  *  ""    "    '.c  • 

a   v  c  r» ,  •*    -,^-v  c  ' 

>  '   •       '         -       •  v       ,  .-l 

:     ^     i.  •  - 


PLATE  11 

Fig.  2.     Highly  magnified  portion  of  myelencephalic  gland  of  Lepiso- 
steus  osseus  from  region  marked  with  circle  in  figure  1.     X  764. 

ABBREVIATIONS 

conn.  tiss.  n. — connective  tissue  network. 
cryth. — erythrocyte? 
gran.  m. — granular  masses. 
leuc. — leucocyte? 


[102] 


UNIV.   CALIF.    PUBL.   ZOOL.  VOL.   9 


[CHANDLER]   PLATE 


conn.  tiss.  n 


leuc. 


gran,  m c. 


eryth. 


PLATE  12 

Fig.  3.  Cross-section  of  myelencephalon  of  21.5  mm.  embryo  of 
Lepisosteus  osseus,  showing  "anlage"  of  myelencephalic  gland.  X  112. 

Fig.  4.  Cross-section  of  myelencephalon  of  55  mm.  embryo  of  Lepi- 
sosteus osseus,  showing  further  development  of  myelencephalic  gland. 
X  85. 

ABBREVIATIONS 
bas.  art. — basilar  artery. 
bl.  sin. — blood  sinus. 
cart. — cartilage. 

conn.  tiss.  n. — connective  tissue  network. 
epend. — ependyma. 
eryth. — erythrocytes. 
med. — medulla  oblongata. 
pig. — pigment  cell. 
4th  vent. — fourth  ventricle. 


[104] 


UNIV,   CALIF.    PUBL.   ZOOL.  VOL.   9 


[CHANDLER]   PLATE  12 


r-  pig 


r eryth. 


.  —  conn.  tiss.  n. 


med. 


NON-CIRCULATING  BOOK 


U.C.  BERKELEY  LIBRARIES 


CQ14DOOED05 


